High-permittivity thin film capacitor for a semiconductor integrated circuit and a fabrication method thereof

ABSTRACT

A semiconductor integrated circuit integrating a high-permittivity thin film capacitor of strontium titanate on the same semiconductor chip, the thin film capacitor consisting of a pair of electrodes and essentially crystalline strontium titanate film therebetween which has a thin surface layer where concentration of titanium is higher than that of the rest of the crystalline strontium titanate film. In one embodiment according to the present invention, a thin film capacitor was fabricated by depositing a 200 nm thick film of strontium titanate at a temperature of 300° C. on a 10 nm thick amorphous titanium oxide film which eventually became the thin surface layer, and subsequent annealing of 250° C. for 30 min in an oxidation atmosphere. A structure and processes realized a thin film capacitor having a dielectric constant of 100 and leakage current density of 4×10 -7  A/cm 2  without degrading characteristics of transistors already fabricated in the same semiconductor chip.

FIELD OF INVENTION

The present invention relates to a high-permittivity thin film capacitor, and more particularly to a high-permittivity thin film capacitor for a semiconductor integrated circuit and a fabrication method thereof.

DESCRIPTION OF THE PRIOR ART

Recently, a capacitor being a smaller in size and larger in an electrostatic capacity has been requested for a high-packing density and high-speed integrated semiconductor circuit, such as a GaAs-IC for a portable telephone and a silicon LSI chip for a dynamic random access memory. For these purposes, crystalline strontium titanate (SrTiO₃) has been particularly paid an attention to because of its dielectric properties such as high permittivity and a low dielectric loss in a high-frequency range. However, a high quality crystalline strontium titanate needs a thermal treatment of higher than 400° C., which is high enough to destroy a microwave monolithic GaAs-IC if a thin film capacitor of crystalline strontium titanate is integrated on the same chip. That simply can not lead to the thermal treatment at a lower temperature because it generally results in degrading crystallinity and increasing leakage current density. For the above reason, a thin film capacitor of crystalline strontium titanate has been mounted on a multichip module discretely from a semiconductor chip in commercial products. An attempt to integrate a thin film capacitor of strontium titanate on a semiconductor chip has recently been done. However, leakage current density is not yet small enough for commercial use.

SUMMARY OF INVENTION

It is an object according to a first aspect of the present invention to provide a thin film of crystalline strontium titanate having high-permittivity, and low leakage characteristics for a capacitor of small size and high electrostatic capacity.

It is an object according to a second aspect of the present invention to provide a semiconductor device comprising a semiconductor integrated circuit and a thin film capacitor of crystalline strontium titanate on the same semiconductor chip.

It is an object according to a third aspect of the present invention to provide a method of fabricating a thin film capacitor of crystalline strontium titanate compatible to a semiconductor integrated circuit.

A representative one of the present invention disclosed hereinafter will be summarized simply hereunder, which is not necessarily limitative.

A high-permittivity thin film essentially consisting of strontium titanate (SrTiO₃) comprising a surface layer where concentration of titanium atoms is higher than that in the rest of the high-permittivity thin film. The film can be obtained by a method comprising steps of depositing an amorphous layer of titanium oxide TiO_(x) (0<X<2) on a conducting layer, and subsequently depositing a layer essentially consisting of strontium titanate (SrTiO₃) on the amorphous layer of titanium oxide. Further, depositing another conducting layer on the layer essentially consisting of strontium titanate (SrTiO₃), annealing the deposited layers in an oxidation atmosphere, and patterning them provides a high-permittivity thin film capacitor according to the present invention.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be more apparent from the following description, when taken to conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a monolithic GaAs-IC having a thin film capacitor of strontium titanate (SrTiO₃) according to the present invention.

FIG. 2 is a diagram illustrating dependence of dielectric constant and leakage current density upon thickness of a thin film of strontium titanate (STO) according to the present invention.

FIG. 3 is a diagram illustrating dependence of dielectric constant and leakage current density upon thickness of an amorphous titanium oxide (TiO_(x)) on which subsequently a thin film of strontium titanate is deposited according to the present invention.

FIG. 4 is a cross-sectional view of a thin film capacitor of strontium titanate (SrTiO₃) integrated on a semiconductor substrate according to the present invention.

FIG. 5 is a circuit diagram of a GaAs MESFET-IC integrating a thin film capacitor of strontium titanate thereon for a portable telephone according to the present invention.

FIG. 6 is a circuit diagram of a memory cell in a silicon MOSIC integrating a thin film capacitor of strontium titanate thereon for a dynamic random access memory according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-sectional view of monolithic GaAs-IC having a thin film capacitor of strontium titanate (SrTiO₃) according to the present invention. A self-aligned gate GaAs-MESFET 30 having refractory metal electrodes for gate 31, source 32, and drain 33 is formed in a GaAs substrate 1, and a thin film capacitor 20 composed of a lower electrode 21, an upper electrode 23, and a high-permittivity thin film of essentially consisting of crystalline strontium titanate 22 between the pair of electrodes is formed on the first insulating layer 2 passivating the surface of the GaAs substrate 1. The GaAs-MESFET 30 and the thin film capacitor 20 are separated by a field insulating layer 5 underlaid by the second insulating layer 6 protecting the side wall of the high-permittivity thin film of essentially consisting of crystalline strontium titanate 22, and connected to an interconnection layer 4 underlaid by a barrier metal layer 3.

Main steps of fabricating the above semiconductor integrated circuit are as follows; After the GaAs-MESFET 30 was fabricated by a conventional method, a 100 nm thick laminated metal layer of platinum and titanium for the lower electrode 21 were successively deposited on the first insulating layer 2 of silicon nitride by sputtering method.

After patterning and etching the metal layer 21, a 10˜50 nm thick amorphous titanium oxide layer TiO_(x) (0<X<2) was deposited by DC reactive sputtering method at a temperature below 200° C. When the temperature is above 200° C., the amorphous titanium oxide layer TiO_(x) is transformed into anatase titanium dioxide (TiO₂) which would considerably degrade permittivity and leakage current density of a thin film composed of TiO₂ +SrTiO₃. The amorphous titanium oxide layer TiO_(x) was confirmed by X-ray diffraction analysis. Subsequently, a thin film of SrTiO₃ was deposited on the amorphous titanium oxide layer TiO_(x) by RF magnetron sputtering method. A sputtering target was a sintered SrTiO₃ powder. Sputtering condition was as follows; RF power density of 1 W/cm², a gas flow rate (Ar/O₂) of 10, a total pressure of 1.2 Pa., a substrate temperature of 300° C., and a deposition thickness of 100˜500 nm thick. The thermal effect during growth of the thin film SrTiO₃ and a subsequent annealing in an oxidation atmosphere at a temperature below 300° C., preferably below 270° C. gave rise to diffusion of strontium atoms into the amorphous titanium layer TiO_(x) resultantly to transform the amorphous titanium layer TiO_(x) into a non-stoichiometric crystalline strontium titanate layer Sr_(1-x) Ti_(x) O₃ (0<X<1). As a whole, a high-permittivity thin film 22 having a low leakage characteristics was obtained. A 100 nm thick platinum layer was deposited on the high-permittivity thin film 22 to form the upper electrode 23 by DC sputtering. Further, a 200 nm thick silicon nitride layer was deposited for a protective layer by plasma CVD at temperature of 300° C. Finally, a titanium-tungsten layer 3 and a gold layer 4 were successively deposited to form the interconnection layer.

FIG. 2 is a diagram illustrating dependence of dielectric constant and leakage current density upon thickness of a thin film of strontium titanate (abbreviated by STO)according to the present invention. It shows that the thin film of strontium titanate deposited on the amorphous titanium oxide film has lower leakage current density and higher dielectric constant throughout the whole thickness measured compared with that deposited directly on a metal surface of an electrode. In the diagram, a solid dot  and an open dot ∘ indicate measured points for each of samples with and without the amorphous titanium oxide film, respectively. For example, a 200 nm thick film of strontium titanate (SrTiO₃) with a 10 nm thick amorphous titanium oxide film (TiO_(x)) has a leakage current density of 9.0×10⁻⁷ A/cm² while a 200 nm thick film of strontium titanate without an amorphous titanium oxide film has a leakage current density of 2.5×10⁻⁵ A/cm², which improves the leakage current density by as much as a factor.

FIG. 3 is a diagram illustrating dependence of dielectric constant indicated by a solid dot  and leakage current density indicated by an open dot ∘ upon thickness of an amorphous titanium oxide (TiO_(x)) on which a thin film of strontium titanate is deposited according to the present invention. It clearly shows that the leakage current density has a minimum point between 10 nm and 30 nm thick. In a typical case, when a 200 nm thick film of strontium titanate was deposited on a 10 nm thick amorphous titanium oxide film, a dielectric constant of 100 and leakage current density of 9×10⁻⁷ A/cm² were obtained. A subsequent annealing of 250° C. for 30 min in an oxidation atmosphere further decreased the leakage current density down to 4×10₋₇ A/cm².

These and other related experiments revealed that the amorphous titanium oxide film should be thinner than 30 nm thick, preferably thinner than 15 nm thick, and further if the amorphous titanium oxide film is thicker than 10 nm thick, the film of strontium titanate should be thicker than 100 nm thick, preferably thicker than 200 nm thick. The optimum deposition temperature of the amorphous titanium oxide film (TiO_(x)) is lower than 220° C., preferably lower than 200° C., while the optimum deposition temperature of the strontium titanate film (SrTiO₃) is between 250° C. and 350° C., preferably between 280° C. and 320° C., and the subsequent annealing in an oxidation atmosphere should be carried out lower than 300° C., preferably lower than 270° C.

Since it is supposed that the thin amorphous titanium oxide film (TiO_(x)) acts as a seed for homogeneous nucleation of a high-quality crystalline strontium titanate film, the thin amorphous strontium titanium oxide film, which may be written by Sr₁ -x Ti_(x) O_(y) (0<X<1, 0<Y<3), is expected to have the same effect on the strontium titanate film (SrTiO₃) to improve leakage characteristics and dielectric constant just as the thin amorphous titanium oxide film has, which also has an advantage in the sputtering process of depositing the strontium titanate film that there is no need to exchange a target of sputtering.

Although strontium titanate SrTiO₃ is paraelectric at a room temperature, it is transformed into ferroelectric by substituting barium atoms for strontium atoms which can be used for a non-volatile memory. Further, considerable improvement in various physical properties such as temperature dependence of dielectric characteristics, moisture resistance, or thermal expansion coefficient as well as dielectric constant itself are generally expected by doping various elements such as other alkaline-earth metals, elements belonging to titanium group and tin group, rare earth metals, or transition metals to the strontium titanate film.

FIG. 4 is a cross-sectional view of a thin film capacitor of strontium titanate (SrTiO₃) 20 integrated on a semiconductor substrate according to the present invention. The dielectric film 22 consists of a strontium titanate (SrTiO₃) 25 and a surface layer 24 composed of the non-stoichiometric crystalline strontium titanate layer Sr_(1-x) Ti_(x) O₃ (0<X<1). It was observed by SIMS analysis that there was the surface layer 24 where indeed a concentration of titanium atoms was higher than that of strontium atoms.

FIG. 5 is a circuit diagram of a GaAs MESFET-IC integrating a thin film capacitor of strontium titanate thereon for a portable telephone according to the present invention, where an integrated circuit 44 consists a GaAs MESFET 40, a load resistance 41, an input resistance 42, and a bypass condenser 43 which is a thin film capacitor.

FIG. 6 is a circuit diagram of a memory cell in a silicon MOSIC integrating a thin film capacitor of strontium titanate thereon for a dynamic random access memory (DRAM) according to the present invention, where the memory cell for DRAM consists of a Si MOSFET 40 for a transfer gate and a thin film capacitor of strontium titanate 43 for a storage capacitor with a bit line 45 and a word line 46. Since a typical dielectric constant of strontium titanate for one embodiment according to the present invention is 100 which is larger almost by a factor than that of silicon dioxide used for the conventional memory cell, therefor, a capacitor area is expected to shrink by a factor that can realize a higher packing density DRAM chip. 

I claim:
 1. A method of fabricating a capacitor having a pair of electrodes and a high-permittivity thin film therebetween comprising the steps of:forming an amorphous layer of titanium oxide TiO_(x) (0<X<2) on a conducting layer for one of the electrodes; and forming a layer essentially consisting of strontium titanate (SrTiO₃) subsequently on the amorphous layer of titanium oxide.
 2. A method of fabricating a capacitor according to claim 1, wherein the amorphous layer of titanium oxide TiO_(x) (0<X<2) is thinner than 30 nm thick.
 3. A method of fabricating a capacitor according to claim 1, wherein the amorphous layer of titanium oxide TiO_(x) (0<X<2) is formed at a temperature below 220° C.
 4. A method of fabricating a capacitor according to claim 1, wherein the layer essentially consisting of titanium oxide strontium (SrTiO₃) is formed at a temperature below 350° C.
 5. A method of fabricating a capacitor according to claim 1, further comprising the step of annealing the amorphous layer of titanium oxide TiO_(x) (0<X<2) and the layer essentially consisting of strontium titanate (SrTiO₃) in an oxidation atmosphere.
 6. A method of fabricating a semiconductor device having a semiconductor integrated circuit essentially formed in a semiconductor substrate and a capacitor consisting a pair of electrodes and a high-permittivity thin film therebetween formed on the semiconductor substrate comprising the steps of:forming an amorphous layer of titanium oxide TiO_(x) (0<X<2) on a conducting layer for one of the electrodes formed on an insulated surface of the semiconductor substrate by keeping a substrate temperature below 220° C.; forming a layer essentially consisting of strontium titanate (SrTiO₃) subsequently on the amorphous layer of titanium oxide keeping the substrate temperature between 250° C. and 350° C. before the amorphous layer of titanium oxide is crystallized; and annealing the amorphous layer of titanium oxide and the layer essentially consisting of strontium titanate in an oxidation atmosphere keeping the substrate temperature below 300° C. so that the amorphous layer of titanium oxide is transformed into a layer having a composition of Sr_(1-x) Ti_(x) O₃ (0<X <1).
 7. A method of fabricating a semiconductor device having a semiconductor integrated circuit essentially formed in a semiconductor substrate and a capacitor consisting a pair of electrodes and a high-permittivity thin film therebetween formed on the semiconductor substrate comprising the steps of:forming an amorphous layer of strontium titanium oxide on a conducting layer for one of the electrodes formed on an insulated surface of the semiconductor substrate by keeping a substrate temperature below 220° C.; forming a layer essentially consisting of strontium titanate (SrTiO₃) subsequently on the amorphous layer of titanium oxide keeping the substrate temperature between 250° C. and 350° C. before the amorphous layer of titanium oxide is crystallized; and annealing the amorphous layer of strontium titanium oxide and the layer essentially consisting of strontium titanate in an oxidation atmosphere keeping the substrate temperature below 300° C. so that the amorphous layer of strontium titanium oxide is transformed into a layer of a crystalline strontium titanate. 